Warm-pressing method of making stacked fuel plates

ABSTRACT

A process for rapidly fabricating large numbers of thin, flat fuel plates of uranium dioxide or uranium dioxide-plutonium dioxide for use in a nuclear reactor. Using U3O8 or an uranium oxide having an oxygen to metal ratio greater than 2 to 1 as starting material, a plurality of such plates are pressed simultaneously in a metallic die at 800* to 1,000* C. in a reducing atmosphere using spacers between the plates.

United States Patent De Freitas et al.

[451 June6, 1972 [54] WARM-PRESSING METHOD OF MAKING STACKED FUEL PLATES[72] Inventors: C. Trench De Freitas, Sao Paulo, Brazil;

Joseph W. Handwerk, Sepulveda, Calif.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission [22] Filed: Oct. 27, 1970 [21] Appl.No.:84,516

[52] U.S. CL... ..264/0.5, 176/67 [51] Int. Cl. ..G21c 21/02, G210 3/18[58] Field of Search ..264/0.5, 322; 176/90, 67

[56] References Cited UNITED STATES PATENTS 3,116,137 12/1963 Vasilosetal ..264/332 2,950,238 8/1960 Nicholson ..264/O.5

Primary Examiner-Benjamin R. Padgett Assistant Examiner-Roger S. GaitherAttorney-Roland A. Anderson 5 7] ABSTRACT A process for rapidlyfabricating large numbers of thin, flat fuel plates of uranium dioxideor uranium dioxide-plutonium dioxide for use in a nuclear reactor. UsingU 0 or an uranium oxide having an oxygen to metal ratio greater than 2to 1 as starting material, a plurality of such plates are pressedsimultaneously in a metallic die at 800 to 1,000" C. in a reducingatmosphere using spacers between the plates.

4 Claims, No Drawings WARMJRESSING METHOD OF MAKING STACKED FUEL PLATECONTRACI'URAL ORIGIN OF THE INVENTION The invention described herein wasmade in the course of, or under, a contract with the United StatesAtomic Energy Commission.

CROSS-REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION Asubstantial effort has gone into development of techniques for thepreparation of ceramic pellets for use as fuel in a nuclear reactor.Cold pressing and sintering is the technique that has been used mostwidely up to this time. Other techniques on which work has been doneinclude hydrostatic pressing, slip casting, extrusion pressing,vibration compaction and swaging. While acceptable results have beenattained using these techniques for preparing small, cylindrical fuelpellets for existing reactor, none of these techniques are completelysatisfactory for preparing a large quantity of thin, flat ceramicplates-of the nature of ceramic tile-such as are expected to be used asfuel in the Zero Power Plutonium Reactor (ZPPR). This reactor willrequire thousands of plates which are square in shape, about 5 cm on aside and about 0.5 cm thick and contain mixed uranium and plutoniumdioxides compressed to a density of 85-87 percent of theoretical. Inparticular, cold pressing and sintering is not completely satisfactorybecause it is difiicult to obtain a pressed compact of completelyuniform thickness and density. This may result in warpage of the tile.In sintering the tile some distortion is likely to be encountered due totemperature variations in the furnace and the nonunifonn presseddensity. In addition, it is difficult to hold the density of thesintered tile within the required limits, since a higher density isnormally obtained by sintering.

Another technique that is available for fabricating ceramic compacts ishot pressing and the above-identified application discloses a practicalmethod for hot pressing simultaneously a large number of such compacts.While the procedure disclosed in this application is perfectly practicaland results in satisfactory ceramic compacts, high temperatures arerequired which entail the use of a graphite die which in turn requiresthat a relatively low die pressure be employed. To overcome the low diepressure, it is necessary that the pressing times be extendedconsiderably in order to obtain the required compact densities. Forexample, the material to be pressed would have to remain in the die forat least 45 minutes and perhaps several hours, depending upon thepressures and temperatures and starting material used, in order toproduce satisfactory fuel plates of sufficient density. If thetemperature could be reduced to no greater than l,l C., metal dies couldbe used which would permit the utilization of higher die pressures andpreparation of the plates would be much faster because residence timesof the material in the dies could be substantially reduced. If theprocedure of the above application be followed employing these lowertemperatures, satisfactory results are not attained since the finaldensity of the compacts is too low.

It is accordingly the object of the present invention to develop amethod of rapidly fabricating a plurality of ceramic compactssimultaneously.

It is also the object of this invention to develop a method of rapidlyfabricating a plurality of ceramic compacts which uses less expensivestarting materials and yet permits the use of lower temperatures than dothe prior art methods.

SUMMARY OF THE INVENTION The inventors have discovered that, by usingrelatively inexpensive U O in place of uranium dioxide and injecting areducing gas into the pressing die, they are able to achieve shorterpressing times using lower temperatures and higher pressures thanpossible with prior art methods.

This can be attained by successively adding accurately weighedincrements of U 0 powder to a vertical metal car- 0 bide die, levelingeach increment, inserting a thin metallic spacer between each of saidincrements, heating to a temperature of 800 to 1,000 C. while pressingat a pressure of from 5,000 to 20,000 psi for a period of about 5minutes while injecting a reducing gas inside the die, removing theplurality of slightly hyperstoichiometric uranium dioxide plates formedfrom the die and maintaining them in a reducing or'inert' at mosphere ata temperature of 800 to 1,000 C. for a period of time sufficient toreduce the hyperstoichiometric uranium dioxide to stoichiometric uraniumdioxide.

DESCRIPTION OF A SPECIFIC EMBODIMENT OF THE INVENTION The hot pressapparatus may consist of an induction-heated furnace mounted in ahydraulic press; a graphite resistor furnace can be used. Pressure andpower can be regulated manually and the temperature measured by means ofan optical pyrometer. The reducing gas which is injected into the dieduring the pressing step may be either hydrogen or carbon monoxide.

The die-susceptor assembly, punches and spacers are machined from ahigh-temperature, high-strength metal carbide such as titanium carbide.Liners made of filter paper can be used in the punch and on the spacerfaces to reduce sticking. While desirable, the use of liners is notbelieved to be essential.

It is important that each increment of powder which is added to the diebe carefully and accurately leveled off in the die before the spacer isinserted in order to produce fuel plates having a uniform thickness.

Although U 0 will give good results in obtaining stoichiometric uraniumdioxide plates, any hyperstoichiometric uranium oxide having an oxygento metal ratio of greater than 2 to 1 can be used with this method.

The hot pressing of the hyperstoichiometric uranium oxide does notresult in complete reduction to U0 00 but results in a slightlyhyperstoichiometric uranium dioxide of U0 Reduction is completed bymaintaining the hot-pressed plates, after removal from the press, in areducing or inert atmosphere such as hydrogen or argon at a temperatureof 800-1,000 C. for a period of time sufficient to complete thereduction to U0 which is about 2 hours.

The hyperstoichiometric uranium oxide powder may be used alone in orderto produce plates of uranium dioxide or it may be mixed with plutoniumdioxide to obtain uranium dioxide-plutonium dioxide fuel plates. If themixed uranium dioxide-plutonium dioxide plates are desired, it isimportant that homogeneity of the powders by achieved by careful powderpreparation before pressing.

The die pressures may vary from 5,000 to 20,000 psi and die temperaturesmay vary from 800 to l,000 C. as necessary in order to obtain thedesired fuel plate densities.

It will be understood that the invention is not to be limited to thedetails given herein but that it may be modified within the scope of theappended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of making a plurality of fuel plates for a nuclear reactorsimultaneously comprising successively adding accurately weighedincrements of hyperstoichiometric uranium oxide to a metal carbide diewhich is oriented vertically, leveling the powder, inserting a metalcarbide spacer between the increments, thereby charging the die, andcompressing the powder to a sufiicient extent to permit the addition ofanother increment after each such incremental addition to charge saiddie, heating said charge to a temperature of 800 to 1,000 C.,compressing the charge at a pressure of 5,000 to 20,000 psi whileinjecting a reducing gas into said die, maintaining said pressure andtemperature for about 5 minutes, thereby forming a plurality of fuelplates of slightly hyperstoichiometric uranium dioxide, discharging saidfuel plates from said die and maintaining said fuel plates in a reducingor inert atmosphere at a temperature of 800 to 1,000 C. for a period oftime sufficient to reduce the slightly 4 hyperstoichiometric uraniumdioxide stoichiometric uranium dioxide.

2. The method of claim 1 wherein the hypeistoichiometric uranium oxideis U 0 3. The method of claim 2 wherein the slightly hyperstoichiometricuranium dioxide fuel plates are maintained in the reducing or inertatmosphere for 2 hours, thereby reducing the plates to uranium dioxide.

4. The method of claim 2 wherein the metal carbide die is titaniumcarbide.

fuel plates to

2. The method of claim 1 wherein the hyperstoichiometric uranium oxideis U3O8.
 3. The method of claim 2 wherein the slightlyhyperstoichiometric uranium dioxide fuel plates are maintained in thereducing or inert atmosphere for 2 hours, thereby reducing the plates touranium dioxide.
 4. The method of claim 2 wherein the metal carbide dieis titanium carbide.